I'm getting this error when I tried to send an email from webmail:
Unable to perform operation. No free disk space.
Note: Main cPanel account and email account doesn't reach the quota.
Solution :-
1) This might be the case that your server's disk space is full which you can verify by firing command df -h through SSH.
2) Check inode usage of your server by firing command df -i through SSH.
3) Check inode usage of that particular cPanel account through cPanel > Statistics section.